With a group III nitride single crystal including aluminum (Al), which has a direct-transition band structure at an ultraviolet range corresponding to 200 nm to 360 nm of wavelength, ultraviolet light emitting devices having high efficiency can be manufactured.
Group III nitride semiconductor devices are generally made by growing a thin film of a group III nitride single crystal on a single crystal substrate, by a chemical vapor deposition such as Metal Organic Chemical Vapor Deposition (MOCVD), Molecular Beam Epitaxy (MBE), and Halide Vapor Phase Epitaxy (HVPE).
For example, a method of: forming a thin film of a first group III nitride single crystal to be a base substrate on a heterogeneous substrate such as sapphire; and growing a thin film of a second group III nitride single crystal on the base substrate is suggested (see Patent Literature 1). Before the thin film of the second group III nitride single crystal is grown, a pretreatment (thermal cleaning) of the base substrate is normally carried out. Specifically, Patent Literature 1 describes, as this pretreatment method (thermal cleaning), a method for removing an oxide film formed on the surface of the substrate, the method including heating the substrate to a temperature of 1050 to 1100° C. under a hydrogen gas atmosphere, keeping the state for a few minutes, thereafter supplying hydrogen gas and ammonia gas, and keeping the state for a few minutes. It is said that this method makes it possible to form a layer of AlGaN (thin film of second group III nitride single crystal) having a flat surface and a high Al content on a base substrate having a layer of aluminum nitride (AlN) single crystal (thin film of first group III nitride single crystal) on its surface.